1. Field of the Invention
The present invention is related to fine wire bonding and the operation of automatic wire bonding machines. More particularly, the present invention is related to a novel method of forming the bonding wire tail under the working face of the fine wire bonding wedge after making a second bond and in preparation for making another first bond.
2. Description of the Prior Art
Fine wire bonding machines are known and were introduced at the time of commercial production of semiconductor devices. Discrete transistor elements in the early 1960's employed fine gold or aluminum wire as connections between electrode pads on the semiconductor chip and a post or lead out pin. Various methods have been employed to separate the wire from the second bonding point after the second bond is made so that the same length of wire is eventually positioned under the working face of the bonding wedge and is ready for making the first bond of a two bond wire interconnection. Manual wedge bonding machines such as those shown in U.S. Pat. Nos. 3,216,640 and 3,543,988 were capable of making a first and second wire bond and subsequently breaking the wire away from the last or second bond. Such manual machines were adapted to feed or move a portion or length of wire through a wire feed guide so that an unshaped length of wire was available for making the next first bond. Sometimes the extended tail or length of wire was performed or shaped by a mechanical device prior to making the next first bond and sometimes the skill of the individual operator was depended upon to inspect for the proper length of the tail and then to form the wire in the process of making the next first bond.
After the introduction of high speed automatic wedge bonding machines which make as many as 120 wire bonds a minute at programmed bonding points, it became impossible to inject operator inspection and responsive solution to assure that the length of tail under the working face of the bonding wedge as well as the shape of the tail was optimum and/or acceptable for making the next first bond.
The problems which effect the length and shape of a wire bonding tail in an automatic wedge bonder are too numerous to enumerate, however, several of the prior art problems which cause the majority of first bond failures will be discussed as a point of departure for the basis of the present novel method.
Something less than ideal tail length and tail position or shape was recognized in the prior art and required continuing adjustment of mechanisms to control tail length and position. Present attempts to make high density VLSI devices reduce the real estate or size of the bonding pads. As the real estate bonding pads size become smaller, smaller wire sizes are employed to make connections. Also, the bonding wedge and the working face of the bonding wedge are made smaller, enabling a bond on smaller bonding pads so that the position and length of the tail under the working face of a bonding wedge is now more critical than it has been in the prior art.
In one type of prior art manual bonding machine, the bonding wedge was held in pressure contact with the second bond when the wire was pulled by an articulating action of a wire clamp to break the wire at the second bond. This causes the free end of the wire in the bonding tool to be elongated beyond its elastic limit and also beyond the yield point at the point adjacent to the second bond which had been purposely deformed and weakened at a cross-sectional area adjacent the second bond. The portion of the wire which was elongated was work hardened, thus stresses were introduced into the wire which caused the wire to curl into an unpredictable shape and assume an unpredictable position.
The manual wedge bonding machine described in U.S. Pat. No. 3,216,640 did not maintain the bonding wedge on the second bond at the time the wire is severed. Instead, the bonding tool was moved first vertically and then horizontally. A remote wire clamp was actuated to place tension in the wire. The length of wire being stretched and the inconsistent angle of pull caused the tail under the bonding tool to be inconsistent.
When articulating wire clamps are employed to clamp the free end of the wire behind the bonding tool and pull the wire to break the wire at the second bond, the clamp may slip on the wire. This results in not pulling an exact length or distance of wire when the wire is broken. When the wire clamp is articulated forward to supply a new length of tail under the working face of the bonding tool, the tail is now too long which often causes an improper first bond even when shaped correctly.
When the articulating wire clamp slips relative to the wire when feeding a new length of wire tail under the working face of the bonding tool, the tail may be too short which results in an improper length of wire under the working face of the bonding tool. This can cause the bond to be squashed out by the predetermined bonding force acting on an insufficient area or can cause an improper bond which easily breaks. The above prior art problems have been recognized, however, it was not generally recognized that the prior art bonding wedge which was in contact with the second bond at the time the wire was pulled to break the wire at the second bond caused a scrubbing action between the bonding wedge and the second bond which noticeably affected the appearance of the second bond. As will be discussed in more detail hereinafter, it was discovered that a scrubbing action occurs at the time of breaking the wire adjacent the second bond which may result in a scrubbing action of the bonding tool on the second bond.
To some extent automatic wire bonders having digital control panels have permitted the operator to program the movement of the bonding tool relative to the bonding position on the semiconductor device more accurately which in turn permits the control of the length of the wire produced when the wire clamping mechanism is operating correctly. This ability to control the length of the bonding tail after the second bond has not solved the problem of inconsistent tail length, inconsistent tail position and shape.